The movie doesn't work without Blake. Actually, the whole trilogy doesn't work without him because in the end Gotham would be left without a worthy individual to take up the Batman mantle (whether he's actually Batman 2.0 or something similar like Nightwing). I've heard some mention that his character seemed too good, but that's exactly the point. Bruce tried giving the reigns of the city to Harvey Dent, a daytime crimefighter who was being more effective, but when the going got tough he fell. With Blake Bruce finally has someone who is relatively pure; like himself but better.